Radio Frequency Nitrogen Plasma Source Research Articles

Growth and characterization of GaAs1−x−ySbxNy/GaAs heterostructures for multijunction solar cell applications

The GaAsSbN dilute-nitride alloy can be grown lattice-matched to GaAs with a bandgap of 1 eV, making it an ideal candidate for use in multijunction solar cells. In this work, using molecular beam epitaxy in conjunction with a radio-frequency nitrogen plasma source, the authors focus first on the growth optimization of the GaAsSb and GaAsN alloys in order to calibrate the Sb and N compositions independently of each other. After the optimum growth conditions to maintain two-dimensional growth were identified, the growth of GaAsSbN films was demonstrated. Both a GaAsSb0.076N0.018/GaAs heterostructure (100 nm thick) and a GaAsSb0.073N0.015/GaAs quantum well (11 nm thick) were grown. X-ray diffraction analysis reveals quite high crystal quality with a small lattice mismatch of 0.13%–0.16%. Secondary ion mass spectrometry profiling revealed that nitrogen was unintentionally incorporated in the GaAs buffer layer during the plasma ignition and stabilization. Nevertheless, a low temperature photoluminescence peak energy of 1.06 eV was measured for the GaAsSbN heterostructure sample while the quantum well emitted photoluminescence at 1.09 eV, which demonstrates promise for realizing 1-eV solar cells.

Surface oxidation of GaN(0001): Nitrogen plasma-assisted cleaning for ultrahigh vacuum applications

The cleaning of metal-organic vapor-phase epitaxial GaN(0001) template layers grown on sapphire has been investigated. Different procedures, performed under ultrahigh vacuum conditions, including degassing and exposure to active nitrogen from a radio frequency nitrogen plasma source have been compared. For this purpose, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and scanning tunneling microscopy have been employed in order to assess chemical as well as structural and morphological surface properties. Initial degassing at 600 °C under ultrahigh vacuum conditions only partially eliminates the surface contaminants. In contrast to plasma assisted nitrogen cleaning at temperatures as low as 300 °C, active-nitrogen exposure at temperatures as high as 700 °C removes the majority of oxide species from the surface. However, extended high-temperature active-nitrogen cleaning leads to severe surface roughening. Optimum results regarding both the removal of surface oxides as well as the surface structural and morphological quality have been achieved for a combination of initial low-temperature plasma-assisted cleaning, followed by a rapid nitrogen plasma-assisted cleaning at high temperature.

Facility for low-temperature spin-polarized-scanning tunneling microscopy studies of magnetic/spintronic materials prepared in situ by nitride molecular beam epitaxy.

Based on the interest in, as well as exciting outlook for, nitride semiconductor based structures with regard to electronic, optoelectronic, and spintronic applications, it is compelling to investigate these systems using the powerful technique of spin-polarized scanning tunneling microscopy (STM), a technique capable of achieving magnetic resolution down to the atomic scale. However, the delicate surfaces of these materials are easily corrupted by in-air transfers, making it unfeasible to study them in stand-alone ultra-high vacuum STM facilities. Therefore, we have carried out the development of a hybrid system including a nitrogen plasma assisted molecular beam epitaxy/pulsed laser epitaxy facility for sample growth combined with a low-temperature, spin-polarized scanning tunneling microscope system. The custom-designed molecular beam epitaxy growth system supports up to eight sources, including up to seven effusion cells plus a radio frequency nitrogen plasma source, for epitaxially growing a variety of materials, such as nitride semiconductors, magnetic materials, and their hetero-structures, and also incorporating in situ reflection high energy electron diffraction. The growth system also enables integration of pulsed laser epitaxy. The STM unit has a modular design, consisting of an upper body and a lower body. The upper body contains the coarse approach mechanism and the scanner unit, while the lower body accepts molecular beam epitaxy grown samples using compression springs and sample skis. The design of the system employs two stages of vibration isolation as well as a layer of acoustic noise isolation in order to reduce noise during STM measurements. This isolation allows the system to effectively acquire STM data in a typical lab space, which during its construction had no special and highly costly elements included, (such as isolated slabs) which would lower the environmental noise. The design further enables tip exchange and tip coating without breaking vacuum, and convenient visual access to the sample and tip inside a superconducting magnet cryostat. A sample/tip handling system is optimized for both the molecular beam epitaxy growth system and the scanning tunneling microscope system. The sample/tip handing system enables in situ STM studies on epitaxially grown samples, and tip exchange in the superconducting magnet cryostat. The hybrid molecular beam epitaxy and low temperature scanning tunneling microscopy system is capable of growing semiconductor-based hetero-structures with controlled accuracy down to a single atomic-layer and imaging them down to atomic resolution.

Cleaning of GaN(2¯110) surfaces

The cleaning of GaN(2¯110) surfaces was investigated by x-ray photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction. Two different two-step cleaning methods, performed under ultrahigh-vacuum conditions, were carried out and compared. The first cleaning step of both methods is thermal degassing. The second step is either the deposition of metallic gallium followed by redesorption or an exposure to active nitrogen from a radio frequency nitrogen plasma source. Upon storage in a glovebox (N2 atmosphere) and transfer to ultrahigh vacuum under dry nitrogen, carbon and oxygen were identified as the major contaminants. A significant decrease in oxygen and carbon was achieved by thermal degassing at 750 °C under ultrahigh-vacuum conditions. By applying a subsequent Ga deposition/redesorption or N2-plasma cleaning step, a further reduction in oxygen and carbon could be achieved. In comparison, the Ga deposition/redesorption cleaning showed a better performance in oxygen removal, whereas the N2 plasma exhibits a better efficiency in carbon removal. Furthermore scanning tunneling microscopy and low-energy electron diffraction investigations showed a drastic improvement of the morphology and atomic structure of the clean surfaces in contrast to the sample surfaces after N2 storage and transfer.

Self-assembled GaInNAs/GaAsN quantum dot lasers: solid source molecular beam epitaxy growth and high-temperature operation

Self-assembled GaInNAs quantum dots (QDs) were grown on GaAs (001) substrate using solid-source molecular-beam epitaxy (SSMBE) equipped with a radio-frequency nitrogen plasma source. The GaInNAs QD growth characteristics were extensively investigated using atomic-force microscopy (AFM), photoluminescence (PL), and transmission electron microscopy (TEM) measurements. Self-assembled GaInNAs/GaAsN single layer QD lasers grown using SSMBE have been fabricated and characterized. The laser worked under continuous wave (CW) operation at room temperature (RT) with emission wavelength of 1175.86 nm. Temperature-dependent measurements have been carried out on the GaInNAs QD lasers. The lowest obtained threshold current density in this work is ∼1.05 kA/cm2from a GaInNAs QD laser (50 × 1,700 µm2) at 10 °C. High-temperature operation up to 65 °C was demonstrated from an unbonded GaInNAs QD laser (50 × 1,060 µm2), with high characteristic temperature of 79.4 K in the temperature range of 10–60 °C.

Initial stage of silicon nitride nucleation on Si(111) by rf plasma-assisted growth

The nucleation of silicon nitride films on Si(111) using a radio frequency nitrogen plasma source has been investigated by scanning tunneling microscopy. The initial nucleation of Si3N4 is always observed at the steps, i.e., either at the step-edges of the initial Si(111) surface or at the step edges of vacancy islands (etch pits) formed on the terrace areas. With increasing nitridation temperature the nitrified patches become larger with lower density and show a triangular shape. After post annealing the triangular nucleation patches at the step-edges disappear and free-standing Si3N4 islands are observed with a hexagonal shape. Nitridation at high temperatures or post-annealing improves the crystalline quality of the nitride films and an atomically resolved honeycomb-like ”8×8” surface reconstruction is observed in STM for thin Si3N4 films grown at 850°C. [DOI: 10.1380/ejssnt.2006.84]

Investigation of GaNAsSb/GaAs and GaInNAsSb/GaNAs/GaAs Band Offsets

AbstractHeterojunction band offsets of GaNAsSb/GaAs, GaInNAsSb/GaAs, and GaInNAsSb/GaNAs/GaAs quantum well (QW) structures were measured by photoreflectance (PR) spectroscopy. These samples were grown by solid-source molecular beam epitaxy using a radio-frequency nitrogen plasma source. PR spectra were collected from the QW structures and the energy transitions were obtained. The experimental data of the QW energy transitions were analyzed by theoretical calculations. Using predetermined values such as QW thickness and composition, unknown factors such as the heterojunction band offsets were able to be determined. For the GaN0.02As0.87Sb0.11/GaAs structure, we found that Qc≈0.5. For Ga0.62In0.38N0.026As0.954Sb0.02/GaAs, we found that Qc≈0.8. This value is similar to the antimony free dilute-nitride material GaInNAs since the small amount of antimony does not affect the band offsets. For the technologically important Ga0.61In0.39N0.023As0.957Sb0.02/GaN0.027As0.973/GaAs laser structure, we found that the GaInNAsSb/GaNAs QW had a conduction band offset of 144 meV and a valence band offset of 127 meV. With a greater understanding of the band structure, more advanced GaInNAsSb laser devices can be obtained.

Comparison of GaNAsSb and GaNAs as quantum-well barriers for GaInNAsSb optoelectronic devices operating at 1.3–1.55μm

Ga N As Sb ∕ Ga As quantum wells were grown by solid-source molecular-beam epitaxy utilizing a radio-frequency nitrogen plasma source. The GaNAsSb layers, originally the quantum well barrier materials for GaInNAs(Sb) devices, were studied for their general growth characteristics as well as their structural and optical properties, which give an indication of its quality as a quantum well barrier material. Reflection high-energy electron diffraction, high-resolution x-ray diffraction, secondary-ion mass spectroscopy, and photoluminescence (PL) measurements were used to study those properties. The growth parameters including arsenic overpressure and substrate temperature were changed systematically to determine the properties during deposition and to optimize these conditions. It was found that the addition of antimony to GaNAs did not improve the material as it did for GaInNAs. PL measurements indicated a decreasing optical quality with an increasing substrate temperature and no change with the arsenic overpressure. In addition, the addition of antimony had eliminated the lattice-strain compensation provided by the GaNAs barriers. Using GaNAs rather than GaNAsSb barriers in the GaInNAs(Sb) devices was found to be advantageous and has dramatically improved the performance of long-wavelength GaAs-based lasers.

Generation of atomic hydrogen during radio-frequency nitrogenplasma-assisted gas-source molecular-beam epitaxy of III-V dilute nitrides

The interaction of a typical gas-source molecular-beam epitaxy (GSMBE) environment with a radio-frequency (RF) nitrogen plasma source is investigated. In particular, a real-time in situ analysis of the evolution of the emission spectrum of an RF nitrogen plasma source, under high partial pressures of hydrogen (∼10−5Torr), is presented. Hydrogen, emanating from the decomposition of hydride precursors in GSMBE, results in the appearance of a sharp emission peak at the region of 656nm in the plasma spectrum, suggesting the generation of atomic hydrogen species in the nitrogen plasma cavity. The intensity of this peak is used for a qualitative evaluation of this interaction and its evolution as a function of the RF nitrogen plasma source conditions is investigated.

Growth of self-assembled GaInNAs quantum dots by atomic-H assisted RF molecular beam epitaxy

Self-organized Ga x In 1− x N y As 1− y quantum dots (QDs) were fabricated on GaAs (001) substrates by atomic hydrogen-assisted molecular beam epitaxy with a radio frequency nitrogen plasma source (RF-MBE). We have investigated the effect of N 2 flow rate on growth dynamics and optical properties of GaInNAs QDs. With increasing N 2 flow rate, both the QD size and size fluctuation became significantly larger possibly due to phase separation and non-uniformity of N atom concentration, thereby causing linewidth broadening of PL emission of band-to-band transition. We found that N 2 flow rate during the growth of GaInNAs QDs on GaAs (001) in RF-MBE strongly affects the growth kinetics and optical properties. Further, appropriate post-growth annealing was found to be effective in improving PL emission efficiency of both thin GaInNAs films and QDs. Finally, we obtained high-quality 1.3-μm wavelength PL emission with a narrow linewidth of 98.4 meV at room temperature from optimally annealed GaInNAs QDs.

Fabrication of self-organized GaInNAs quantum dots by atomic H-assisted RF-molecular beam epitaxy

Self-organized Ga x In 1− x N y As 1− y quantum dots (QDs) were fabricated on GaAs (0 0 1) substrates by atomic hydrogen-assisted molecular beam epitaxy with a radio frequency nitrogen plasma source (RF-MBE). By adjusting the amount of lattice mismatch between Ga x In 1− x N y As 1− y and GaAs to be almost the same as that of In 0.4Ga 0.6As and GaAs, we were able to fabricate Ga 0.53In 0.47N 0.02As 0.98 QDs with densities up to ∼1.1×10 11 cm −2 on GaAs (0 0 1) by atomic H-assisted RF-MBE. Further, we have studied the effect of atomic H irradiation on QDs’ growth dynamics and their optical properties. Based on the atomic force microscope and photoluminescence measurements, we found that there exists an optimum range of H 2 flow rates for a uniform and high quality Ga 0.53In 0.47N 0.02As 0.98 QDs formation on GaAs substrates.

Studies on single- and multi-layer InAsN quantum dots grown by solid-source molecular beam epitaxy

InAsN quantum dots were grown on GaAs (0 0 1) substrate by a solid-source molecular beam epitaxy system equipped with a radio-frequency nitrogen plasma source. The quantum dot formation was confirmed by reflection high-energy electron diffraction observation and atomic force microscopy measurement. Dot density as high as ∼1×10 11 cm −2 was achieved. Low-temperature and room-temperature photoluminescence measurements were carried out to investigate the optical property of the quantum dots. Multilayer quantum dot structures were found to shift the photoluminescence emission peak to longer wavelength from 1025 to 1120 nm.

Incorporation of N into GaAsN under N overpressure and underpressure conditions

GaAsN was grown by molecular beam epitaxy equipped with a radio frequency nitrogen plasma source. The N incorporation behaviors were investigated using a series of samples grown at different growth rates, As4/Ga ratios, and nitrogen fluxes within a growth temperature range from 420 to 560 °C. It was found that, for the GaAsN grown at higher growth rates (0.4–1.3 ML/s), the N concentration increased linearly following a decrease in growth rate, and kept independent of the arsenic pressure. For the GaAsN grown at lower growth rate (0.1–0.3 ML/s), the N concentration was significantly influenced by the arsenic pressure, and the increase in N concentration with decreasing growth rate follows a sublinear manner. We propose a model based on the incorporation competition of group V elements (N and As) under N underpressure and overpressure conditions, which can explain the different behaviors well.

Thickness effects on properties of GaInNAs quantum dots grown by molecular-beam epitaxy with nitrogen plasma source

Self-assembled GaInNAs quantum dots (QDs) were grown on a GaAs (001) substrate by solid-source molecular-beam epitaxy equipped with a radio-frequency nitrogen plasma source. The GaInNAs QD growth characteristics were investigated using atomic-force microscopy (AFM) and photoluminescence (PL) measurements. The growth experiment was conducted using indium and nitrogen compositions of 50% and ∼1%, respectively, at different nominal thicknesses of 3 monolayers (MLs) to 6 MLs. AFM images suggest the onset of QD formation at ∼3 MLs and QD coalescence at ∼6 MLs. The highest PL intensity at low temperature (5 K) was obtained from the sample with nominal thickness of 5 ML, consistent with the highest QD density observed from AFM measurement. Under the present experimental conditions, GaIn0.5N0.01As QDs of 5–6 MLs gave the best optical property in terms of the lowest full width at half maximum and highest PL intensity. The reduced barrier potential of a strain-compensated layer (SCL) of GaNAs inserted between the substrate and GaInNAs QD layer was able to extend the PL emission wavelength by more than 140 nm compared to the sample without the GaNAs SCL.

Optical property of self-assembled GaInNAs quantum dots grown by solid source molecular beam epitaxy

Self-assembled GaInNAs/GaAs quantum dots (QDs) are promising structures for extending the emission wavelength of GaInNAs/GaAs quantum wells from 1.3 to 1.55 μm and beyond. We report herein the growth of GaInNAs/GaAs quantum dot samples of different deposited thickness by solid source molecular beam epitaxy using active nitrogen radicals generated by a radio frequency nitrogen plasma source. Images from atomic force spectroscopy reveal the increase of non-uniformity QDs size following the increase in the deposited thickness. Temperature-dependent photoluminescence (PL) measurements show PL line width shrinkage at low temperature suggests the relaxation of carriers into neighboring QD local-energy minimum. The low thermal activation energy of ∼72 meV, estimated from the temperature-dependent integrated PL intensity curve suggests the existence of non-radiative recombination centers that quench the luminescence intensity at higher temperature.

Photoluminescence characteristics of GaInNAs quantum wells annealed at high temperature

The photoluminescence (PL) characteristics of GaInNAs quantum wells (QWs) after high-temperature postgrowth annealing were studied. The QWs were grown using a radio-frequency nitrogen plasma source in conjunction with a solid-source molecular-beam epitaxy system. It was found that annealing at high temperature (840 °C) and long duration (10 min) results in significant improvements to the PL characteristics of the GaInNAs QWs. The shift of the GaInNAs and GaInAs PL peak wavelength resulting from high-temperature annealing is dependent on the In composition. It is suggested that the dominant mechanisms that give rise to the blueshift of the PL peak wavelength in GaInNAs QWs with high-In composition are residual-strain-induced GaAs/GaInNAs/GaAs interface interdiffusion, and defect-assisted diffusion-related effects, both of which originate from the growth process.

Correlation of the structural and optical properties of GaN grown on vicinal (0 0 1) GaAs substrates with the plasma-assisted MBE growth conditions

Molecular beam epitaxy, using a radio frequency nitrogen plasma source, was investigated for the direct growth of GaN thin films on vicinal (0 0 1) GaAs substrates. It has been found possible to grow GaN thin films with various types of crystal structure. Cubic (0 0 1) GaN with epitaxial relationship to the GaAs substrate could be grown without GaAs nitridation and under stoichiometric N/Ga flux ratio conditions. Hexagonal mixing in the cubic GaN films occurred mainly as domains with the [0 0 0 1] oriented parallel to the GaAs surface normal. Their origin was related to irregularities at the GaAs surface, produced by interaction with the N-beam. Layers which exhibited intense 17 K photoluminescence with the main peak at approximately 3.42 eV, presented a polycrystalline hexagonal structure with a domain width of several 100 nm. These layers did not exhibit a “yellow-band” luminescence.

MBE growth of different hexagonal GaN crystal structures on vicinal (100) GaAs substrates

GaN thin films of different hexagonal crystal structures have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy (RFMBE) on vicinal (100) GaAs substrates. Polycrystalline hexagonal material occurred for high temperature (630°C) nitridation of the GaAs surface or low temperatures of the initial GaN buffer layer deposition. On the contrary, initial GaN growth at 540°C gave hexagonal single crystals with [0001] axis either inclined at approximately 43° from the growth axis or aligned parallel to it. The GaN orientation depended on the annealing or not, respectively, of the initial low temperature buffer layer.

Single-crystal hexagonal and cubic GaN growth directly on vicinal (001) GaAs substrates by molecular-beam epitaxy

Single-crystal hexagonal and cubic GaN thin films have been grown by radio-frequency nitrogen plasma source molecular beam epitaxy directly on vicinal (001) GaAs substrates, misoriented by 2° toward [100], without using an incident As beam during oxide desorption or the following stages of growth. Both the GaAs nitridation and GaN growth conditions were found to control the structure of the layers. Cubic layers could be grown only without nitridation and under stoichiometric N/Ga flux ratio conditions. N-rich conditions favored the growth of hexagonal layers, which exhibited significantly higher photoluminescence intensities compared to cubic ones. Hexagonal single crystalline GaN films were grown with (101̄2) planes and presented characteristic surface roughness striations along a 〈110〉 substrate direction. On the contrary, a stepped surface morphology was observed for cubic GaN.

The (3 × 3) reconstruction and its evolution during the nitridation of GaAs(001)

Nitridation of GaAs(001) was performed using a radio frequency nitrogen plasma source in a molecular beam epitaxy chamber. The nitrogen-induced GaAs(001) (3 × 3) reconstruction was investigated by reflection high energy electron diffraction (RHEED) and X-ray photoemission spectroscopy (XPS). The temperature dependence of the (3 × 3) reconstruction as well as its evolution during further nitridation was observed by RHEED; we found that the (3 × 3) reconstruction could be obtained in the temperature range 400–580 °C by a very low dose of atomic nitrogen deposition. The nitrogen coverage in the (3 × 3) reconstruction is determined to be in the range 0.2–0.4 ML using XPS. We also discuss the atomic model of the (3 × 3) reconstruction and the energetics of its formation and stability.